1. Field of the Invention
The present invention relates to a displacement control actuator which has a relatively large displacement while maintaining the high reliability and is capable of controlling displacement with great precision.
2. Description of the Related Art
Conventional piezoelectric actuators are classified into a linear displacement type and a flexional displacement type: the linear displacement type being classified into a single-plate type and a laminated type, and the flexional displacement type including a bimorph type. Though the linear displacement type piezoelectric actuator, of the laminated type in particular, cannot provide very large displacement, it is used as a fine motion actuator for precision machines, etc. since it can provide strong power. However, this piezoelectric displacement actuator is apt to be an expensive actuator since it is made by laminating a large number of piezoelectric elements and electrodes. Further, the flexional displacement type piezoelectric displacement actuator of the bimorph type is made by bonding mechanical-electrical converter elements to one another and bonding the mechanical-electrical converter elements to metal plates. This actuator is utilized in a large number of fields since it has an extremely large displacement and is inexpensive.
Speaking concretely, a bimorph type mechanical-electrical converter element 50 of the bimorph type actuator is made by bonding piezoelectric ceramic plates 51a and 51b on which electrodes 52a and 52b are formed with a bonding agent 53 such as an epoxy resin as shown in FIG. 28. Further, the bimorph type mechanical-electrical converter element 50 has a cantilever structure wherein an end of the bimorph type mechanical-electrical converter element 50 is bonded and fixed to a fixing member 55 with an electrically conductive bonding agent 54 or the like as shown in FIG. 29. Since resonance frequencies of bimorph type mechanical-electrical converter elements which have the cantilever structure vary from converter element to converter element, these mechanical-electrical converter elements are employed in fields where they are used at low frequencies far from the resonance frequencies and other fields where driving frequencies can be adequately selected for individual converter elements. As an example where a piezoelectric ceramic is used in a light deflector, there is conventionally known a light deflector wherein a mirror is attached to an actuator which is made by laminating piezoelectric elements and a direction of the mirror is changed by applying a voltage to the actuator. (V. J. Fowler & J. Schlafer. Proc. IEEE., VOL. 54 (1966), p. 1437). Due to the fact that the light deflector uses the laminated type actuator, however, it has the drawback that it cannot provide a large deflection angle relative to the applied voltage.
Further, another light deflector (Japanese Patent Laid-Open No. 58-95710) rotates a mirror by utilizing bimorph type actuators. However, this light deflector has the drawback that it has an extremely complicated structure due to the fact that a plurality of bimorph type actuators are coupled mechanically with a rotating shaft of the mirror.
Furthermore, still another light deflector (Japanese Patent Laid-Open No. 58-189618) is configured so as to divide an electrode for piezoelectric elements of a bimorph type actuator into a plurality of sections, and control the deformation degree of the piezoelectric elements by controlling the number of electrodes to which a voltage is applied. However, this light deflector has the drawback that the control of the degree of deflection is complicated.
A precision displacement control actuator which uses such a piezoelectric ceramic poses the problem that it cannot well control a displacement because it exhibits a remarkable non-linearity in relationship between applied voltages and displacements at applied voltages higher than 10% of a breakdown limit, even when the driving frequency is set at a level far lower than the resonance frequency of the piezoelectric elements. Another problem is that the displacement of the precision displacement control actuator is largely variable, since the piezoelectric ceramic per se is manufactured by mixing and burning various material, and has a material constant larger than that of a single crystal material.
Furthermore, such a conventional actuator poses the problem that it reduces displacement since it ordinarily uses, for bonding the piezoelectric ceramic, a bonding agent containing an epoxy resin or the like which has a Young's modulus not exceeding 0.5.times.10.sup.10 N/m.sup.2 far smaller than the Young's modulus of 5.times.10.sup.10 N/m.sup.2 to 15.times.10.sup.10 N/m.sup.2 of the piezoelectric ceramic and absorbs distortion of the mechanical-electrical converter element caused by the application of the driving voltage.
Moreover, such a conventional actuator poses still another problem in that it allows the characteristics of the mechanical-electrical converter element, displacement and resonance frequency, to be variable since it is difficult to bond the piezoelectric ceramic with a layer of the bonding agent having uniform thickness.
For stabilizing the displacement of a rectangular bimorph type piezoelectric converter element, it is additionally necessary to stabilize its resonance frequency. Though it is necessary to stabilize the fixed condition of the mechanical-electrical converter element for this purpose, a portion of the bimorph type mechanical-electrical converter element which is supported or fixed with a supporting or fixing member made of a metal or the like deviates due to stresses produced by mechanical or temperature variations. When the mechanical-electrical converter element is fixed with a bonding agent, for example, its fixed position is changed depending on the application ranges of the bonding agents, thereby varying the resonance frequency of the mechanical-electrical converter element. Further, it is difficult to maintain the stable fixed condition of the mechanical-electrical converter element since the fixed condition varies by temperature the variations of the bonding agent.
The light deflector which utilizes such a mechanical-electrical converter element also poses problems in that it requires a high driving voltage for obtaining a large deflection angle due to the characteristics of the mechanical-electrical converter element described above. Another problem is that it allows a deflection angle of the light deflector to vary markedly.